linux-kernel - RE: [RFC 1/2] kvm: host-side changes for tmem on KVM

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Date:	Thu, 15 Mar 2012 12:51:43 -0700 (PDT)
From:	Dan Magenheimer <dan.magenheimer@...cle.com>
To:	Akshay Karle <akshay.a.karle@...il.com>,
	Konrad Wilk <konrad.wilk@...cle.com>
Cc:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	ashu tripathi <er.ashutripathi@...il.com>,
	nishant gulhane <nishant.s.gulhane@...il.com>,
	Shreyas Mahure <shreyas.mahure@...il.com>,
	amarmore2006 <amarmore2006@...il.com>,
	mahesh mohan <mahesh6490@...il.com>
Subject: RE: [RFC 1/2] kvm: host-side changes for tmem on KVM

> From: Akshay Karle [mailto:akshay.a.karle@...il.com]
> Subject: Re: [RFC 1/2] kvm: host-side changes for tmem on KVM
> 
> >> @@ -669,7 +670,6 @@ static struct zv_hdr *zv_create(struct x
> >>       int chunks = (alloc_size + (CHUNK_SIZE - 1)) >> CHUNK_SHIFT;
> >>       int ret;
> >>
> >> -     BUG_ON(!irqs_disabled());
> >
> > Can you explain why?
> 
> Zcache is by default used in the non-virtualized environment for page compression. Whenever
> a page is to be evicted from the page cache the spin_lock_irq is held on the page mapping.
> To ensure that this is done, the BUG_ON(!irqs_disabled()) was used.
> But now the situation is different, we are using zcache functions for kvm VM's.
> So if any page of the guest is to be evicted the irqs should be disabled in just that
> guest and not the host, so we removed the BUG_ON(!irqs_disabled()); line.

I think irqs may still need to be disabled (in your code by the caller)
since the tmem code (in tmem.c) takes spinlocks with this assumption.
I'm not sure since I don't know what can occur with scheduling a
kvm guest during an interrupt... can a different vcpu of the same guest
be scheduled on this same host pcpu?

Dan
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